The present invention relates to a surgical resection guide. In particular, the present invention relates to a surgical resection guide incorporating a mechanism for adjusting the spacing between two components which are coupled together.
During a total knee replacement procedure it is necessary to resect the end of the tibia to prepare the end of the tibia for implantation of a tibial prosthesis component. It may be necessary to resect a significant amount of bone from the end of the tibia, for instance 10 mm of bone. It is necessary to ensure that the resection plane is accurately located. In particular, it is necessary to ensure that the resection plane is positioned along the longitudinal axis of the tibia at a predetermined distance from a reference point on the surface of the end of the tibia.
A tibial resection is typically performed by aligning a cutting tool with a plane of a cutting guide. The cutting tool may press against and slide along an exterior surface of the cutting guide or the cutting tool may pass through a cutting slot formed within the cutting guide. Before the resection can be performed, the cutting guide must be accurately positioned relative to the bone such that the cutting guide exterior surface or the cutting slot is aligned with the chosen resection plane. Once in position, the cutting guide may be temporarily pinned in position to ensure a stable platform during the resection.
In order to adjust the position of the cutting guide the cutting guide may be mounted upon a slide rail coupled to the patient's leg and aligned with the tibia. The position of the cutting guide may then be adjusted along the rail until it is determined that it is correctly aligned with the resection plane. In order to align the cutting guide with a required resection plane it is known to provide a tibial stylus which couples to the cutting guide to allow the position of the cutting guide to be set relative to the reference point on the end of the bone.
Referring to FIG. 1, this illustrates a known form of tibial stylus. The tibial stylus comprises a shaft 2 coupled to a foot 4. The foot 4 comprises two plates 6a, 6b which extend from opposite sides of the shaft 2. Each plate 6a, 6b may be inserted into the cutting slot formed within the cutting guide. The plates 6a, 6b extend from shaft 2 at offset positions along the longitudinal axis of the shaft 2. The offset of plates 6a, 6b allows the same tibial stylus to be used to adjust the position of the cutting guide relative to the reference point on the end of the stylus either when the resection is to be performed through the cutting slot or when is it to be performed by aligning the cutting tool with an exterior surface of the cutting guide, according to the preference of the surgeon. If the resection is to be performed by aligning the cutting tool with an exterior surface of the cutting guide closer to the end of the bone then the cutting slot then plate 6a should be inserted into the cutting slot. If the chosen surgical technique is for resection to be performed through the cutting slot then plate 6b should be inserted into the cutting guide. The difference in position between plates 6a, 6b along shaft 2 is equal to the distance on the cutting guide between the exterior surface of the cutting guide and the cutting slot.
The tibial stylus further comprises a housing 8 having a bore through which the shaft 2 can slide. The housing 8 may also rotate about the shaft 2. The tibial stylus further comprises a stylus arm 10 coupled to the housing 8 so that it can slide through the housing 8 along an axis transverse to the axis of the shaft 2. Preferably the axis of the stylus arm 10 is perpendicular to the axis of the shaft 2.
The shaft 2 further comprises a series of annular grooves 12 proximal to the foot 4. Coupled to the housing 8 is a lever arm 14. Lever arm 14 is pivotally mounted upon the housing 8 and is biased by spring 16 such that a lower tooth part of the lever arm 14 engages an annular groove 12. When the lever arm 14 bears against an annular groove 12 the engagement of the tooth part with the annular groove 12 limits the ability of the shaft 2 to slide through the housing 8. While it may prove possible to overcome the engagement of the lever arm 14 with groove 12 by applying a force to the housing while holding the foot in position, the engagement is sufficiently strong that this is unlikely to happen accidently during a surgical procedure and in any event would be apparent to the surgeon as the tooth part clicked into a new groove. The strength of the engagement between the lever arm 14 and groove 12 is dependent upon the respective shapes of the groove 12 and the tooth part and the depth of the groove. The lever arm 14 may be released from an annular groove 12 by squeezing the free end of the lever arm 14 towards the housing 8, thereby overcoming the force applied to the lever arm 14 by spring 16. The shaft 2 is then free to slide through the housing 8 until the free end of the lever arm 14 is released and engages a different groove 12.
The tip 18 of stylus arm 10 may thus be raised or lowered relative to the foot 4. The tip 18 may be located on the reference point on the end of the bone by rotating the housing, and thus the stylus arm 10, around the shaft 2 and by sliding the stylus arm 10 though the housing 8. By adjusting the spacing between the tip 18 and the foot 4 parallel to the axis of shaft 2, by raising and lowering housing 8 along shaft 2 as described above, the cutting guide may be positioned relative to the bone so that the bone may be resected at an appropriate point along the bone away from the reference point. The exact location of a required resection plane is dependent upon an individual patient's anatomy.
The tibial stylus illustrated in FIG. 1 is calibrated so that the offset along the bone from the reference point to the resection plane is indicated by a gauge 20 engraved on the housing 8. The shaft 2 includes a reference mark 22 which indicates the position on the gauge. Sliding motion of the shaft 2 through the housing 8 adjusts the offset between the reference point and the resection plane. Sliding motion of the shaft 2 is limited to predetermined increments, for instance 2 mm, which are equal to the distance between adjacent annular grooves 12 along the axis of the shaft 2.
The known tibial stylus illustrated in FIG. 1 suffers from the disadvantage that the size of the markings on the gauge 20 are necessarily small, even if the numbering is staggered on the gauge 20, for instance the numbering may be at most 2.5 mm high for annular grooves 12 around the shaft which are 2 mm wide. This can mean that the gauge 20 is hard to read.
A further limitation of the known tibial stylus is that in order to adjust the position of the housing 8 along shaft 2 the lever arm 14 must be squeezed against the housing 8 and then the housing 8 raised or lowered along the shaft 2. This raising and lowering requires the use of large motor groups for the surgeon (arms and shoulders to lift the housing) in order to control a fine movement, which can result in overshooting the required resection level.